How do I zero out the pressure and make the adjustments to set the regulator pressure?

Assuming you still have the stock regulator, use a 5/8" open end wrench or adjustable crescent wrench and turn the bottom nut (it's the wide, black one. Not the macroline fitting) counter-clockwise (viewed from the bottom of the regulator) until it stops to turn it all the way down.

Then, while using a chronograph, turn it clockwise to increase the pressure until you are firing at 270-300fps.